This invention relates to a phosphor screen formed by depositing a phosphor layer on a substrate consisting of a fiber plate, and a method of manufacturing said phosphor screen.
An image tube containing a phosphor screen, such as an X-ray image intensifier, is mainly applied in medical uses, though it is also used in an industrial X-ray television designed for industrial nondestructive examination.
The above-mentioned X-ray image intensifier is constructed as illustrated, for example, in FIG. 1. An input screen 2 is set, on the input side, within a vacuum envelope 1. An anode 3 and output screen 4 are provided, on the output side, within said glass vacuum envelope 1. A focusing electrode 5 extends along the inner lateral wall of the vacuum envelope 1. The input screen 2 comprises a spherical aluminum substrate 6, an input phosphor layer 7 prepared from CsI and stretched along the output side (concave plane) of said substrate 6, and a photocathode 8 formed on the surface of said phosphor layer 7. The output screen 4 is formed of a substrate 9 and an output phosphor layer 10 settled on the surface of said substrate 9.
The X-ray image intensifier constructed as described above is operated in the following manner. An X-ray beam penetrating a foregoing subject and modulated in accordance with the magnitude of the X-ray transmittance of said foreground subject enters the X-ray image intensifier to excite the input phosphor layer 7. A light generated by said excitation energizes the photocathode 8, which in turn issues electrons. The released electrons are accelerated by the action of an electron lens comprised of an anode 3 and focusing electrode 5 and focused on the output phosphor layer 10, which in turn irradiates a light. The above-mentioned process amplifies the electrons. Thus, a light image decidedly brighter than the light image obtained by the input phosphor layer 7 is released from the output phosphor layer 10.
Japanese Patent Application Disclosure No. 53-24,770 discloses an X-ray image intensifier of the above-mentioned type, which is characterized in that contrast is improved by forming an output phosphor layer on an optical fiber plate. As shown in FIG. 2, an output screen 16 consists of an optical fiber plate 17 and an output phosphor layer 10 deposited on said optical fiber plate 17, and is placed on the output side within the vacuum envelope 1. The above-mentioned construction of the output screen 16 makes it impossible to directly draw out an image signal from the vacuum envelope, unlike the arrangement in which the optical fiber plate is used as part of the vacuum envelope, and therefore requires the application of a lens system. However, the proposed X-ray image intensifier has an advantage in that an accelerating voltage can be impressed in the same manner as in the X-ray image intensifier shown in FIG. 1. Nevertheless, the device proposed in said Japanese patent application disclosure No. 53-24,770 also has drawbacks in that the improvement in the image contrast still remains unsatisfactory. The reason for this is given below. FIG. 3 illustrates the manner in which light reflection taken place within the optical filber. The optical fiber consists of a core 101 and clad 102. Let us assume that n.sub.1 denotes the refractive index of the core 101, n.sub.2 represents the refractive index of the clad 102, and n.sub.0 shows the refractive index of a vacuum. Then, the maximum value of an incident angle .theta..sub.0 with respect to the optical fiber, which is required to assure the transmission of a light through the optical fiber, by repeating total reflection, may be expressed as follows: ##EQU1## For the sake of description, let it be assumed that n.sub.1 equals 1.8, and n.sub.2 equals 1.49. In such a case, the incident angle .theta..sub.0 is determined, from the above equation to be about 90.degree.. This means that all light rays entering the optical fiber from the region of the vacuum are transmitted through said optical fiber. To confirm this event concretely, the refractive angle .theta..sub.1 of a light ray entering the core 101 at an angle of, e.g., 90.degree. is determined to be 33.7.degree. from the equation, where n.sub.1 sin .theta..sub.2 =n.sub.2 sin .theta..sub.0. The critical angle .theta..sub.2 of total reflection at the boundary between the core 101 and clad 102 is determined to be 55.9.degree. from the equation, where n.sub.1 sin .theta..sub.2 =n.sub.2 sin .theta..sub.3 (.theta..sub.3 =90.degree.). An incident angle .phi..sub.1 of a light ray having a refractive angle .theta..sub.1 of 33.7.degree. with respect to the boundary between the core 101 and clad 102 is 90.degree.-33.7.degree., which equals 56.3.degree.; a value larger than the aforementioned critical angle. Therefore, the light ray is transmitted through the fiber by repeating total reflection, without leaking into the adjacent fiber, and is finally brought to the opposite plane of the fiber to that plane thereof at which the light enters. An outgoing angle .theta..sub.2 of the light equals the incident angle .theta..sub.0 of the light.
When, however, a phosphor layer is deposited over an optical fiber plate, a noticeable change occurs in the above-process of light transmission. The manner in which the light is transmitted through the fiber plate 17 may now be described with reference to FIG. 4. The phosphor layer 10 is generally formed by attaching phosphor particles 201 to the surface of the fiber plate 17 by means of a vitreous bonding agent. The fiber plate 17 and phosphor particles 201 are in firm contact with each other, as optically viewed. Accordingly, the light having 33.7.degree. of .theta..sub.1 in FIG. 3, incident to the central axis of the core 101 through the vitreous bonding agent layer from the phosphor particles without passing via the space, is transmitted from the emitting surface of the core 101 at 90.degree.. In other words, there exists a light which is emitted at a wide angle of 0.degree. to 90.degree. on the emitting surface of the core 101 irrespective of the degree of the contact of the phosphor layer 10 with the optical fiber plate 17.
On the boundary surface between substances of a different refraction index, there exists a light which is reflected on the boundary surface at the same angle as the incident angle, except the light which passes through the boundary emerges as the refracted light. This phenomenon is called "Fresnel's reflection". Fresnel's reflection is largely affected by the incident angle. The relationship between the incident angle and the reflectance of the reflected light by the Fresnel's reflection is shown in FIG. 5. In FIG. 5, curves a and b illustrate the reflectance of the reflected light by Fresnel's reflection generated when the light is incident from a vacuum into a glass, curves c and d illustrate the reflectance of the reflected light by Fresnel's reflection generated when the light is incident in a glass into the air. Curves a and c illustrate components on the incident surface which includes the incident light and a vertical line to the boundary surface of the reflectance, and curves b and d illustrate components on the plane vertical to the incident surface of the reflectance. As apparent from FIG. 5, the reflectance of the reflected light by Fresnel's reflection becomes vigorously large as the incident angle increases.
The light A emitted from the fiber plate 17, as shown in FIG. 4, is reflected on the incident surface and the emitting surface of the output window 18 by the influence of the abovementioned Fresnel's reflection, and returned as light rays B, C to the fiber plate 17. These lights, such as light ray B, are observed to be generated from the phosphor particles different from the phosphor particle initially generated, and the contrast accordingly decreases.